1. Field of the Invention
The present invention relates generally to receiver devices, and more particularly, to a receiver device that can quickly suppress the adverse effect of multipath reflection interference. The invention has particular applicability to FM receivers incorporated in mobile units such as an automobile.
2. Description of the Background Art
A radio receiver or a radio broadcasting receiver carried within a mobile unit such as an automobile receives signals having a field strength varying in accordance with the location of the mobile unit. Regarding a radio broadcasting receiver incorporated in an automobile, the field strength of the reception signal varies as the automobile moves. The present invention can generally be applied to a receiver device provided in a mobile unit. A radio broadcasting receiver provided in an automobile will be taken as an example in the description hereinafter.
The field strength of the reception signal received by a radio receiver in an automobile varies in accordance with the travel of the automobile. When the automobile is between a large structure such as a building, the field strength of the reception signal is reduced significantly. Thus, noise components in the reception signal suddenly increase to generate unpleasant noise through the speaker of the radio receiver.
A radio receiver is often provided with a muting circuit, called a "soft muting circuit," for the purpose of suppressing the generation of such noise. This muting circuit gradually reduces the level of the sound signal generated from the receiver as the field strength of the reception signal becomes weaker. This prevents the generation of unpleasant noise through the speaker.
Referring to FIG. 1, curves C1 and C2 in solid lines show the sound signal level and the noise level in the case where muting processing is not applied. Curves C3 and C4 in single dot dashed lines respectively show the sound signal level and the noise level in the case where muting processing is applied. It can be appreciated from the figure that in the case where muting processing is not applied, the levels of the sound signal and the noise are both not reduced, that is to say, not muted, when the field strength of the reception signal is low (curves C1 and C2). In the case where muting processing is applied, the levels of the sound signal and the noise are both reduced, that is to say muted, when the field strength of the sound signal is low (curves C3 and C4). By applying muting processing to the sound signal, the level of the sound generated from the speaker is lowered in accordance with the field strength of the reception signal to eliminate the unpleasant noise caused by the noise.
A conventional soft muting circuit operates gradually as the field strength of the reception signal is lowered. Therefore, when the field strength of the reception signal suddenly changes such as in the case of multipath reflection interference, the function of muting could not operate so quickly. Therefore, when the electric field of the reception signal suddenly changes due to multipath reflection interference, noise unpleasant to the ear was generated from the speaker because the noise could not be suppressed.
The above mentioned multipath reflection interference is generated as follows. Because radio waves are reflected by buildings and mountains, reception signals include reflected waves arriving through various paths, in addition to the transmitted radio waves directly sent from a transmitter. Interference occurs between the received radio waves to increase the noise level.